Rob Duffy headed Grimsby to a first win of the season against ten-man Cambridge.
The visitors had already seen Harrison Dunk sent off for a late challenge when Grimsby went ahead, Tyrone Thompson nodding in a Frankie Artus corner.
The U's levelled when Michael Wylde slammed home from a corner.
But Duffy won the game when he headed in another Artus corner, although Grimsby survived a scare when James Jennings hit the post.
Grimsby: McKeown, Wood, Pearson, Kempson, Ridley, Thompson (Coulson 59), Disley, Artus, Makofo, Duffy (Elding 83), Hearn. Subs Not Used: Church, Spencer, I'Anson.
Booked: Disley, Pearson.
Goals: Thompson 28, Duffy 64.
Cambridge Utd: Naisbitt, Roberts, Coulson, Wylde, Jennings, Carew (Platt 71), Jarvis (Charles 76), Berry, Shaw, Dunk, Gash. Subs Not Used: Brown, McAuley, Kinniburgh.
Sent Off: Dunk (19).
Booked: Berry, Roberts, Jennings, Jarvis.
Goal: Wylde 30.
Referee: Stephen Smith (County Durham).